1. Field of the Invention
This invention pertains to an apparatus and method for furling and reefing a sail.
2. Description of the Related Art
The roller furler concept originated in earlier generation of sail powered vessels as a method of managing staysails. A staysail is generally a triangular shaped sail whose leading edge is supported by a flexible cable or rod under tension. Staysails are known by various names such as, but not limited to Jibs, Genoas, Yankees, Gennekers, Code 0s, In-mast Furling Mainsails, and the like. A roller furler is a mechanical device which winds the staysail around the tensioned edge cable.
The original concept was conceived to provide two functions. The first function, known as ‘furling’, provides storage of the sail when not deployed. This aspect of the concept allows a simple method to set or strike a staysail. Prior art has produced many concepts which serve this function. The second function, known as ‘reefing’, was seen as an additional benefit. Reefing is the common term for an operation which partially stores a sail and thereby reduces the amount of a sail exposed to an air stream. The purpose of this reduction in sail area is to reduce the power being generated by the sail commensurate with stability of the sailing vessel. It is common for sailing vessels to be overwhelmed by excess wind pressure.
Prior art for the reefing function has not been entirely successful. Sails are constructed with a three dimensional curvature to produce precise airfoil shapes with highly efficient lift-to-drag ratio when properly supported and exposed to an air stream. This three dimensional curvature presents geometric difficulties for prior art. Rolling a three dimensional curved surface around an essentially straight stay cable normally causes severe distortion of the efficient airfoil shape. Past attempts to reef a sail have produced such distorted sail aerodynamic properties that the sail no longer produces the necessary efficient lift to propel the boat in a close-hauled angle of motion. This loss in close-hauled (upwind) performance reduces the vessel's operational safety margins when navigating in proximity to obstructions or in inclement weather.
Prior art devices generally wind a sail from a mechanism driving a more or less torsionally rigid hollow sail mounting track fitted over the stay cable. This rigid hollow sail mounting track rotates the entire luff edge of the sail in a uniform motion, (i.e. all at once). The result of this method is to generate shear stresses in the sail as it is partially rolled (reefed). This shear stress causes sail material to accumulate in the midsection of the sail, thereby producing exaggerated camber and severe distortion in the airfoil shape.
Prior art devices have included upper swivel assemblies that insufficiently maintain alignment and position engagement with the sail mounting tracks. This upper swivel device provides an attachment between the sail halyard and the sail. To accommodate the winding of the sail, the swivel device must allow the sail to swivel freely around the sail mounting track and or the leading edge support cable with the halyard being tensioned. As the sail is wound or furled, the sail attachment portion of the swivel device rotates. When rotation occurs, the load on the swivel device becomes skewed causing torque inputs that increase friction and may eventually damage the contact points (typically in the sail mounting tracks). This increased friction, may also cause a condition known as “Halyard wrap”, where the upper swivel device does not “rotationally isolate” the sail from the halyard and where, during winding, the halyard is undesirably wound around the stay cable.
A second failure mode is when the sail mounting track becomes positioned inadequately high on the stay cable. During operation, the upper swivel device slides along the sail-mounting track until the sail is fully hoisted. If the resulting position of a fully hoisted sail and upper swivel extends beyond the height of the sail-mounting track, it can become disengaged. When this occurs, the sail cannot be lowered and the boat becomes in jeopardy.